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Internet Engineering Task Force (IETF) J. Klensin Request for Comments: 6530 Y. Ko Obsoletes: 4952, 5504, 5825 February 2012 Category: Standards Track ISSN: 2070-1721

         Overview and Framework for Internationalized Email

Abstract

 Full use of electronic mail throughout the world requires that
 (subject to other constraints) people be able to use close variations
 on their own names (written correctly in their own languages and
 scripts) as mailbox names in email addresses.  This document
 introduces a series of specifications that define mechanisms and
 protocol extensions needed to fully support internationalized email
 addresses.  These changes include an SMTP extension and extension of
 email header syntax to accommodate UTF-8 data.  The document set also
 includes discussion of key assumptions and issues in deploying fully
 internationalized email.  This document is a replacement for RFC
 4952; it reflects additional issues identified since that document
 was published.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6530.

Copyright Notice

 Copyright (c) 2012 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect

Klensin & Ko Standards Track [Page 1] RFC 6530 Internationalized Email Framework February 2012

 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Klensin & Ko Standards Track [Page 2] RFC 6530 Internationalized Email Framework February 2012

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
 2.  Role of This Specification . . . . . . . . . . . . . . . . . .  4
 3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  5
 4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.1.  Mail User and Mail Transfer Agents . . . . . . . . . . . .  6
   4.2.  Address Character Sets . . . . . . . . . . . . . . . . . .  7
   4.3.  User Types . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.5.  Mailing Lists  . . . . . . . . . . . . . . . . . . . . . .  8
   4.6.  Conventional Message and Internationalized Message . . . .  8
   4.7.  Undeliverable Messages, Notification, and Delivery
         Receipts . . . . . . . . . . . . . . . . . . . . . . . . .  8
 5.  Overview of the Approach and Document Plan . . . . . . . . . .  9
 6.  Review of Experimental Results . . . . . . . . . . . . . . . .  9
 7.  Overview of Protocol Extensions and Changes  . . . . . . . . . 10
   7.1.  SMTP Extension for Internationalized Email Address . . . . 10
   7.2.  Transmission of Email Header Fields in UTF-8 Encoding  . . 11
   7.3.  SMTP Service Extension for DSNs  . . . . . . . . . . . . . 12
 8.  Downgrading before and after SMTP Transactions . . . . . . . . 12
   8.1.  Downgrading before or during Message Submission  . . . . . 13
   8.2.  Downgrading or Other Processing after Final SMTP
         Delivery . . . . . . . . . . . . . . . . . . . . . . . . . 14
 9.  Downgrading in Transit . . . . . . . . . . . . . . . . . . . . 15
 10. User Interface and Configuration Issues  . . . . . . . . . . . 15
   10.1. Choices of Mailbox Names and Unicode Normalization . . . . 15
 11. Additional Issues  . . . . . . . . . . . . . . . . . . . . . . 17
   11.1. Impact on URIs and IRIs  . . . . . . . . . . . . . . . . . 17
   11.2. Use of Email Addresses as Identifiers  . . . . . . . . . . 17
   11.3. Encoded Words, Signed Messages, and Downgrading  . . . . . 18
   11.4. Other Uses of Local Parts  . . . . . . . . . . . . . . . . 18
   11.5. Non-Standard Encapsulation Formats . . . . . . . . . . . . 19
 12. Key Changes from the Experimental Protocols and Framework  . . 19
 13. Security Considerations  . . . . . . . . . . . . . . . . . . . 19
 14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   15.1. Normative References . . . . . . . . . . . . . . . . . . . 21
   15.2. Informative References . . . . . . . . . . . . . . . . . . 22

Klensin & Ko Standards Track [Page 3] RFC 6530 Internationalized Email Framework February 2012

1. Introduction

 In order to use internationalized email addresses, it is necessary to
 internationalize both the domain part and the local part of email
 addresses.  The domain part of email addresses is already
 internationalized [RFC5890], while the local part is not.  Without
 the extensions specified in this document, the mailbox name is
 restricted to a subset of 7-bit ASCII [RFC5321].  Though MIME
 [RFC2045] enables the transport of non-ASCII data, it does not
 provide a mechanism for internationalized email addresses.  In RFC
 2047 [RFC2047], MIME defines an encoding mechanism for some specific
 message header fields to accommodate non-ASCII data.  However, it
 does not permit the use of email addresses that include non-ASCII
 characters.  Without the extensions defined here, or some equivalent
 set, the only way to incorporate non-ASCII characters in any part of
 email addresses is to use RFC 2047 coding to embed them in what RFC
 5322 [RFC5322] calls the "display name" (known as a "name phrase" or
 by other terms elsewhere) of the relevant header fields.  Information
 coded into the display name is invisible in the message envelope and,
 for many purposes, is not part of the address at all.
 This document is a replacement for RFC 4952 [RFC4952]; it reflects
 additional issues, shared terminology, and some architectural changes
 identified since that document was published.  It obsoletes that
 document.  The experimental descriptions of in-transit downgrading
 [RFC5504] [RFC5825] are now irrelevant and no longer needed due to
 the changes discussed in Section 12.  The RFC Editor is requested to
 move all three of those documents to Historic.
 The pronouns "he" and "she" are used interchangeably to indicate a
 human of indeterminate gender.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC 2119
 [RFC2119].

2. Role of This Specification

 This document presents the overview and framework for an approach to
 the next stage of email internationalization.  This new stage
 requires not only internationalization of addresses and header
 fields, but also associated transport and delivery models.  A prior
 version of this specification, RFC 4952 [RFC4952], also provided an
 introduction to a series of experimental protocols [RFC5335]
 [RFC5336] [RFC5337] [RFC5504] [RFC5721] [RFC5738] [RFC5825].  This
 revised form provides overview and conceptual information for the
 Standards Track successors of a subset of those protocols.  Details

Klensin & Ko Standards Track [Page 4] RFC 6530 Internationalized Email Framework February 2012

 of the documents and the relationships among them appear in Section 5
 and a discussion of what was learned from the experimental protocols
 and their implementations appears in Section 6.
 Taken together, these specifications provide the details for a way to
 implement and support internationalized email.  The document itself
 describes how the various elements of email internationalization fit
 together and the relationships among the primary specifications
 associated with message transport, header formats, and handling.
 This document, and others that comprise the collection described
 above, assume a reasonable familiarity with the basic Internet
 electronic mail specifications and terminology [RFC5321] [RFC5322]
 and the MIME [RFC2045] and 8BITMIME [RFC6152] ones as well.  While
 not strictly required to implement this specification, a general
 familiarity with the terminology and functions of IDNA [RFC5890]
 [RFC5891] [RFC5892] [RFC5893] [RFC5894] are also assumed.

3. Problem Statement

 Internationalizing Domain Names in Applications (IDNA) [RFC5890]
 permits internationalized domain names, but deployment has not yet
 reached most users.  One of the reasons for this is that we do not
 yet have fully internationalized naming schemes.  Domain names are
 just one of the various names and identifiers that are required to be
 internationalized.  In many contexts, until more of those identifiers
 are internationalized, internationalized domain names alone have
 little value.
 Email addresses are prime examples of why it is not good enough to
 just internationalize the domain name.  As most observers have
 learned from experience, users strongly prefer email addresses that
 resemble names or initials to those involving seemingly meaningless
 strings of letters or numbers.  Unless the entire email address can
 use familiar characters and formats, users will perceive email as
 being culturally unfriendly.  If the names and initials used in email
 addresses can be expressed in the native languages and writing
 systems of the users, the Internet will be perceived as more natural,
 especially by those whose native language is not written in a subset
 of a Roman-derived script.
 Internationalization of email addresses is not merely a matter of
 changing the SMTP envelope; or of modifying the "From:", "To:", and
 "Cc:" header fields; or of permitting upgraded Mail User Agents
 (MUAs) to decode a special coding and respond by displaying local
 characters.  To be perceived as usable, the addresses must be
 internationalized and handled consistently in all of the contexts in
 which they occur.  This requirement has far-reaching implications:

Klensin & Ko Standards Track [Page 5] RFC 6530 Internationalized Email Framework February 2012

 collections of patches and workarounds are not adequate.  Even if
 they were adequate, a workaround-based approach may result in an
 assortment of implementations with different sets of patches and
 workarounds having been applied with consequent user confusion about
 what is actually usable and supported.  Instead, we need to build a
 fully internationalized email environment, focusing on permitting
 efficient communication among those who share a language and writing
 system.  That, in turn, implies changes to the mail header
 environment to permit those header fields that are appropriately
 internationalized to utilize the full range of Unicode characters, an
 SMTP extension to permit UTF-8 [RFC3629] [RFC5198] mail addressing
 and delivery of those extended header fields, support for
 internationalization of delivery and service notifications [RFC3461]
 [RFC3464], and (finally) a requirement for support of the 8BITMIME
 SMTP extension [RFC6152] so that all of these can be transported
 through the mail system without having to overcome the limitation
 that header fields do not have content-transfer-encodings.

4. Terminology

 This document assumes a reasonable understanding of the protocols and
 terminology of the core email standards as documented in RFC 5321
 [RFC5321] and RFC 5322 [RFC5322].

4.1. Mail User and Mail Transfer Agents

 Much of the description in this document depends on the abstractions
 of "Mail Transfer Agent" ("MTA") and "Mail User Agent" ("MUA").
 However, it is important to understand that those terms and the
 underlying concepts postdate the design of the Internet's email
 architecture and the application of the "protocols on the wire"
 principle to it.  That email architecture, as it has evolved, and
 that "on the wire" principle have prevented any strong and
 standardized distinctions about how MTAs and MUAs interact on a given
 origin or destination host (or even whether they are separate).
 However, the term "final delivery MTA" is used in this document in a
 fashion equivalent to the term "delivery system" or "final delivery
 system" of RFC 5321.  This is the SMTP server that controls the
 format of the local parts of addresses and is permitted to inspect
 and interpret them.  It receives messages from the network for
 delivery to mailboxes or for other local processing, including any
 forwarding or aliasing that changes envelope addresses, rather than
 relaying.  From the perspective of the network, any local delivery
 arrangements such as saving to a message store, handoff to specific
 message delivery programs or agents, and mechanisms for retrieving
 messages are all "behind" the final delivery MTA and hence are not
 part of the SMTP transport or delivery process.

Klensin & Ko Standards Track [Page 6] RFC 6530 Internationalized Email Framework February 2012

4.2. Address Character Sets

 In this document, an address is "all-ASCII", or just an "ASCII
 address", if every character in the address is in the ASCII character
 repertoire [ASCII]; an address is "non-ASCII", or an "i18n-address",
 if any character is not in the ASCII character repertoire.  Such
 addresses MAY be restricted in other ways, but those restrictions are
 not relevant to this definition.  The term "all-ASCII" is also
 applied to other protocol elements when the distinction is important,
 with "non-ASCII" or "internationalized" as its opposite.
 The umbrella term to describe the email address internationalization
 specified by this document and its companion documents is "SMTPUTF8".
 For example, an address permitted by this specification is referred
 to as a "SMTPUTF8 (compliant) address".
 Please note that, according to the definitions given here, the set of
 all "all-ASCII" addresses and the set of all "non-ASCII" addresses
 are mutually exclusive.  The set of all addresses permitted when
 SMTPUTF8 appears is the union of these two sets.

4.3. User Types

 An "ASCII user" (i) exclusively uses email addresses that contain
 ASCII characters only, and (ii) cannot generate recipient addresses
 that contain non-ASCII characters.
 An "internationalized email user" has one or more non-ASCII email
 addresses, or is able to generate recipient addresses that contain
 non-ASCII characters.  Such a user may have ASCII addresses too; if
 the user has more than one email account and a corresponding address,
 or more than one alias for the same address, he or she has some
 method to choose which address to use on outgoing email.  Note that
 under this definition, it is not possible to tell from an ASCII
 address if the owner of that address is an internationalized email
 user or not.  (A non-ASCII address implies a belief that the owner of
 that address is an internationalized email user.)  There is no such
 thing as an "internationalized email user message"; the term applies
 only to users and their agents and capabilities.  In particular, the
 use of non-ASCII, and hence presumably internationalized, message
 content is an integral part of the MIME specifications [RFC2045] and
 does not require these extensions (although it is compatible with
 them).

Klensin & Ko Standards Track [Page 7] RFC 6530 Internationalized Email Framework February 2012

4.4. Messages

 A "message" is sent from one user (the sender) using a particular
 email address to one or more other recipient email addresses (often
 referred to just as "users" or "recipient users").

4.5. Mailing Lists

 A "mailing list" is a mechanism whereby a message may be distributed
 to multiple recipients by sending it to one recipient address.  An
 agent (typically not a human being) at that single address then
 causes the message to be redistributed to the target recipients.
 This agent sets the envelope return address of the redistributed
 message to a different address from that of the original single
 recipient message.  Using a different envelope return address
 (reverse-path) causes error (and other automatically generated)
 messages to go to an error-handling address.
 Special provisions for managing mailing lists that might contain non-
 ASCII addresses are discussed in a document that is specific to that
 topic [RFC5983] and its expected successor [RFC5983bis-MailingList].

4.6. Conventional Message and Internationalized Message

 o  A conventional message is one that does not use any extension
    defined in the SMTP extension document [RFC6531] or in the
    UTF8header document [RFC6532] in this set of specifications, and
    is strictly conformant to RFC 5322 [RFC5322].
 o  An internationalized message is a message utilizing one or more of
    the extensions defined in this set of specifications, so that it
    is no longer conformant to the traditional specification of an
    email message or its transport.

4.7. Undeliverable Messages, Notification, and Delivery Receipts

 As specified in RFC 5321, a message that is undeliverable for some
 reason is expected to result in notification to the sender.  This can
 occur in either of two ways.  One, typically called "Rejection",
 occurs when an SMTP server returns a reply code indicating a fatal
 error (a "5yz" code) or persistently returns a temporary failure
 error (a "4yz" code).  The other involves accepting the message
 during SMTP processing and then generating a message to the sender,
 typically known as a "Non-delivery Notification" or "NDN".  Current
 practice often favors rejection over NDNs because of the reduced
 likelihood that the generation of NDNs will be used as a spamming
 technique.  The latter, NDN, case is unavoidable if an intermediate
 MTA accepts a message that is then rejected by the next-hop server.

Klensin & Ko Standards Track [Page 8] RFC 6530 Internationalized Email Framework February 2012

 A sender MAY also explicitly request message receipts [RFC3461] that
 raise the same issues for these internationalization extensions as
 NDNs.

5. Overview of the Approach and Document Plan

 This set of specifications changes both SMTP and the character
 encoding of email message headers to permit non-ASCII characters to
 be represented directly.  Each important component of the work is
 described in a separate document.  The document set, whose members
 are described below, also contains Informational documents whose
 purpose is to provide implementation suggestions and guidance for the
 protocols.
 In addition to this document, the following documents make up this
 specification and provide advice and context for it.
 o  SMTP extension.  The SMTP extension document [RFC6531] provides an
    SMTP extension (as provided for in RFC 5321) for internationalized
    addresses.
 o  Email message headers in UTF-8.  The email message header document
    [RFC6532] essentially updates RFC 5322 to permit some information
    in email message headers to be expressed directly by Unicode
    characters encoded in UTF-8 when the SMTP extension described
    above is used.  This document, possibly with one or more
    supplemental ones, will also need to address the interactions with
    MIME, including relationships between SMTPUTF8 and internal MIME
    headers and content types.
 o  Extensions to delivery status and notification handling to adapt
    to internationalized addresses [RFC6533].
 o  Forthcoming documents will specify extensions to the IMAP protocol
    [RFC3501] to support internationalized message headers
    [RFC5738bis-IMAP], parallel extensions to the POP protocol
    [RFC5721] [RFC5721bis-POP3], and some common properties of the two
    [POPIMAP-downgrade].

6. Review of Experimental Results

 The key difference between this set of protocols and the experimental
 set that preceded them [RFC5335] [RFC5336] [RFC5337] [RFC5504]
 [RFC5721] [RFC5738] [RFC5825] is that the earlier group provided a
 mechanism for in-transit downgrading of messages (described in detail
 in RFC 5504).  That mechanism permitted, and essentially required,
 that each non-ASCII address be accompanied by an all-ASCII
 equivalent.  That, in turn, raised security concerns associated with

Klensin & Ko Standards Track [Page 9] RFC 6530 Internationalized Email Framework February 2012

 pairing of addresses that could not be authenticated.  It also
 introduced the first incompatible change to Internet mail addressing
 in many years, raising concerns about interoperability issues if the
 new address forms "leaked" into legacy email implementations.  After
 examining experience with the earlier, experimental, predecessors of
 these specifications, the working group that produced them concluded
 that the advantages of in-transit downgrading, were it feasible
 operationally, would be significant enough to overcome those
 concerns.
 That turned out not to be the case, with interoperability problems
 among initial implementations.  Prior to starting on the work that
 led to this set of specifications, the WG concluded that the
 combination of requirements and long-term implications of that
 earlier model were too complex to be satisfactory and that work
 should move ahead without it.
 The other significant change to the protocols themselves is that the
 SMTPUTF8 keyword is now required as an SMTP client announcement if
 the extension is needed; in the experimental version, only the server
 announcement that an extended envelope and/or content were permitted
 was necessary.

7. Overview of Protocol Extensions and Changes

7.1. SMTP Extension for Internationalized Email Address

 An SMTP extension, "SMTPUTF8", is specified as follows:
 o  Permits the use of UTF-8 strings in email addresses, both local
    parts and domain names.
 o  Permits the selective use of UTF-8 strings in email message
    headers (see Section 7.2).
 o  Requires that the server advertise the 8BITMIME extension
    [RFC6152] and that the client support 8-bit transmission so that
    header information can be transmitted without using a special
    content-transfer-encoding.
 Some general principles affect the development decisions underlying
 this work.
 1.  Email addresses enter subsystems (such as a user interface) that
     may perform charset conversions or other encoding changes.  When
     the local part of the address includes characters outside the
     ASCII character repertoire, use of ASCII-compatible encoding
     (ACE) [RFC3492] [RFC5890] in the domain part is discouraged to

Klensin & Ko Standards Track [Page 10] RFC 6530 Internationalized Email Framework February 2012

     promote consistent processing of characters throughout the
     address.
 2.  An SMTP relay MUST
  • Either recognize the format explicitly, agreeing to do so via

an ESMTP option, or

  • Reject the message or, if necessary, return a non-delivery

notification message, so that the sender can make another

        plan.
 3.  If the message cannot be forwarded because the next-hop system
     cannot accept the extension, it MUST be rejected or a non-
     delivery message MUST be generated and sent.
 4.  In the interest of interoperability, charsets other than UTF-8
     are prohibited in mail addresses and message headers being
     transmitted over the Internet.  There is no practical way to
     identify multiple charsets properly with an extension similar to
     this without introducing great complexity.
 Conformance to the group of standards specified here for email
 transport and delivery requires implementation of the SMTP extension
 specification and the UTF-8 header specification.  If the system
 implements IMAP or POP, it MUST conform to the internationalized IMAP
 [RFC5738bis-IMAP] or POP [RFC5721bis-POP3] specifications
 respectively.

7.2. Transmission of Email Header Fields in UTF-8 Encoding

 There are many places in MUAs or in a user presentation in which
 email addresses or domain names appear.  Examples include the
 conventional "From:", "To:", or "Cc:" header fields; "Message-ID:"
 and "In-Reply-To:" header fields that normally contain domain names
 (but that may be a special case); and in message bodies.  Each of
 these must be examined from an internationalization perspective.  The
 user will expect to see mailbox and domain names in local characters,
 and to see them consistently.  If non-obvious encodings, such as
 protocol-specific ACE variants, are used, the user will inevitably,
 if only occasionally, see them rather than "native" characters and
 will find that discomfiting or astonishing.  Similarly, if different
 codings are used for mail transport and message bodies, the user is
 particularly likely to be surprised, if only as a consequence of the
 long-established "things leak" principle.  The only practical way to
 avoid these sources of discomfort, in both the medium and the longer
 term, is to have the encodings used in transport be as similar to the
 encodings used in message headers and message bodies as possible.

Klensin & Ko Standards Track [Page 11] RFC 6530 Internationalized Email Framework February 2012

 When email local parts are internationalized, they SHOULD be
 accompanied by arrangements for the message headers to be in the
 fully internationalized form.  That form SHOULD use UTF-8 rather than
 ASCII as the base character set for the contents of header fields
 (protocol elements such as the header field names themselves are
 unchanged and remain entirely in ASCII).  For transition purposes and
 compatibility with legacy systems, this can be done by extending the
 traditional MIME encoding models for non-ASCII characters in headers
 [RFC2045] [RFC2231], but even these should be based on UTF-8, rather
 than other encodings, if at all possible [RFC6055].  However, the
 target is fully internationalized message headers, as discussed in
 [RFC6532] and not an extended and painful transition.

7.3. SMTP Service Extension for DSNs

 The existing Delivery Status Notifications (DSNs) specification
 [RFC3461], which is a Draft Standard, is limited to ASCII text in the
 machine-readable portions of the protocol.  "International Delivery
 and Disposition Notifications" [RFC6533] adds a new address type for
 international email addresses so an original recipient address with
 non-ASCII characters can be correctly preserved even after
 downgrading.  If an SMTP server advertises both the SMTPUTF8 and the
 DSN extension, that server MUST implement internationalized DSNs
 including support for the ORCPT parameter specified in RFC 3461
 [RFC3461].

8. Downgrading before and after SMTP Transactions

 An important issue with these extensions is how to handle
 interactions between systems that support non-ASCII addresses and
 legacy systems that expect ASCII.  There is, of course, no problem
 with ASCII-only systems sending to those that can handle
 internationalized forms because the ASCII forms are just a proper
 subset.  But, when systems that support these extensions send mail,
 they MAY include non-ASCII addresses for senders, receivers, or both
 and might also provide non-ASCII header information other than
 addresses.  If the extension is not supported by the first-hop system
 (i.e., the SMTP server accessed by the submission server acting as an
 SMTP client), message-originating systems SHOULD be prepared to
 either send conventional envelopes and message headers or to return
 the message to the originating user so the message may be manually
 downgraded to the traditional form, possibly using encoded words
 [RFC2047] in the message headers.  Of course, such transformations
 imply that the originating user or system must have ASCII-only
 addresses available for all senders and recipients.  Mechanisms by
 which such addresses may be found or identified are outside the scope

Klensin & Ko Standards Track [Page 12] RFC 6530 Internationalized Email Framework February 2012

 of these specifications as are decisions about the design of
 originating systems such as whether any required transformations are
 made by the user, the originating MUA, or the submission server.
 A somewhat more complex situation arises when the first-hop system
 supports these extensions but some subsequent server in the SMTP
 transmission chain does not.  It is important to note that most cases
 of that situation with forward-pointing addresses will be the result
 of configuration errors: especially if it hosts non-ASCII addresses,
 a final delivery MTA that accepts these extensions SHOULD NOT be
 configured with lower-preference MX hosts that do not.  When the only
 non-ASCII address being transmitted is backward-pointing (e.g., in an
 SMTP MAIL command), recipient configuration cannot help in general.
 On the other hand, alternate, all-ASCII addresses for senders are
 those most likely to be authoritatively known by the submission
 environment or the sender herself.  Consequently, if an intermediate
 SMTP relay that requires these extensions then discovers that the
 next system in the chain does not support them, it will have little
 choice other than to reject or return the message.
 As discussed above, downgrading to an ASCII-only form may occur
 before or during the initial message submission.  It might also occur
 after the delivery to the final delivery MTA in order to accommodate
 message stores, IMAP or POP servers, or clients that have different
 capabilities than the delivery MTA.  These cases are discussed in the
 subsections below.

8.1. Downgrading before or during Message Submission

 The IETF has traditionally avoided specifying the precise behavior of
 MUAs to provide maximum flexibility in the associated user
 interfaces.  The SMTP standard [RFC5321], Section 6.4, gives wide
 latitude to MUAs and submission servers as to what might be supplied
 by the user as long as the result conforms with "on the wire"
 standards once it is injected into the public Internet.  In that
 tradition, the discussion in the remainder of Section 8 is provided
 as general guidance rather than normative requirements.
 Messages that require these extensions will sometimes be transferred
 to a system that does not support these extensions; it is likely that
 the most common cases will involve the combination of ASCII-only
 forward-pointing addresses with a non-ASCII backward-pointing one.
 Until the extensions described here have been universally implemented
 in the Internet email environment, senders who prefer to use non-
 ASCII addresses (or raw UTF-8 characters in header fields), even when
 their intended recipients use and expect all-ASCII ones, will need to
 be especially careful about the error conditions that can arise.  The

Klensin & Ko Standards Track [Page 13] RFC 6530 Internationalized Email Framework February 2012

 risks are especially great in environments in which non-delivery
 messages (or other indications from submission servers) are routinely
 dropped or ignored.
 Perhaps obviously, the most convenient time to find an ASCII address
 corresponding to an internationalized address is at the originating
 MUA or closely associated systems.  This can occur either before the
 message is sent or after the internationalized form of the message is
 rejected.  It is also the most convenient time to convert a message
 from the internationalized form into conventional ASCII form or to
 generate a non-delivery message to the sender if either is necessary.
 At that point, the user has a full range of choices available,
 including changing backward-pointing addresses, contacting the
 intended recipient out of band for an alternate address, consulting
 appropriate directories, arranging for translation of both addresses
 and message content into a different language, and so on.  While it
 is natural to think of message downgrading as optimally being a fully
 automated process, we should not underestimate the capabilities of a
 user of at least moderate intelligence who wishes to communicate with
 another such user.
 In this context, one can easily imagine modifications to message
 submission servers (as described in RFC 6409 [RFC6409]) so that they
 would perform downgrading operations or perhaps even upgrading ones.
 Such operations would permit receiving messages with one or more of
 the internationalization extensions discussed here and adapting the
 outgoing message, as needed, to respond to the delivery or next-hop
 environment the submission server encounters.

8.2. Downgrading or Other Processing after Final SMTP Delivery

 When an email message is received by a final delivery MTA, it is
 usually stored in some form.  Then it is retrieved either by software
 that reads the stored form directly or by client software via some
 email retrieval mechanisms such as POP or IMAP.
 The SMTP extension described in Section 7.1 provides protection only
 in transport.  It does not prevent MUAs and email retrieval
 mechanisms that have not been upgraded to understand
 internationalized addresses and UTF-8 message headers from accessing
 stored internationalized emails.
 Since the final delivery MTA (or, to be more specific, its
 corresponding mail storage agent) cannot safely assume that agents
 accessing email storage will always be capable of handling the
 extensions proposed here, it MAY downgrade internationalized emails,
 specially identify messages that utilize these extensions, or both.
 If either or both of these actions were to be taken, the final

Klensin & Ko Standards Track [Page 14] RFC 6530 Internationalized Email Framework February 2012

 delivery MTA SHOULD include a mechanism to preserve or recover the
 original internationalized forms without information loss.
 Preservation of that information is necessary to support access by
 SMTPUTF8-aware agents.

9. Downgrading in Transit

 The base SMTP specification (Section 2.3.11 of RFC 5321 [RFC5321])
 states that "due to a long history of problems when intermediate
 hosts have attempted to optimize transport by modifying them, the
 local-part MUST be interpreted and assigned semantics only by the
 host specified in the domain part of the address".  This is not a new
 requirement; equivalent statements appeared in specifications in 2001
 [RFC2821] and even in 1989 [RFC1123].
 Adherence to this rule means that a downgrade mechanism that
 transforms the local part of an email address cannot be utilized in
 transit.  It can only be applied at the endpoints, specifically by
 the MUA or submission server or by the final delivery MTA.
 One of the reasons for this rule has to do with legacy email systems
 that embed mail routing information in the local part of the address
 field.  Transforming the email address destroys such routing
 information.  There is no way a server other than the final delivery
 server can know, for example, whether the local part of
 user%foo@example.com is a route ("user" is reached via "foo") or
 simply a local address.

10. User Interface and Configuration Issues

 Internationalization of addresses and message headers, especially in
 combination with variations on character coding that are inherent to
 Unicode, may make careful choices of addresses and careful
 configuration of servers and DNS records even more important than
 they are for traditional Internet email.  It is likely that, as
 experience develops with the use of these protocols, it will be
 desirable to produce one or more additional documents that offer
 guidance for configuration and interfaces.  A document that discusses
 issues with MUAs, especially with regard to downgrading, is expected
 to be developed.  The subsections below address some other issues.

10.1. Choices of Mailbox Names and Unicode Normalization

 It has long been the case that the email syntax permits choices about
 mailbox names that are unwise in practice, if one actually intends
 the mailboxes to be accessible to a broad range of senders.  The most
 often cited examples involve the use of case-sensitivity and tricky
 quoting of embedded characters in mailbox local parts.  These

Klensin & Ko Standards Track [Page 15] RFC 6530 Internationalized Email Framework February 2012

 deliberately unusual constructions are permitted by the protocols,
 and servers are expected to support them.  Although they can provide
 value in special cases, taking advantage of them is almost always bad
 practice unless the intent is to create some form of security by
 obscurity.
 In the absence of these extensions, SMTP clients and servers are
 constrained to using only those addresses permitted by RFC 5321.  The
 local parts of those addresses MAY be made up of any ASCII characters
 except the control characters that RFC 5321 prohibits, although some
 of them MUST be quoted as specified there.  It is notable in an
 internationalization context that there is a long history on some
 systems of using overstruck ASCII characters (a character, a
 backspace, and another character) within a quoted string to
 approximate non-ASCII characters.  This form of internationalization
 was permitted by RFC 821 [RFC0821] but is prohibited by RFC 5321
 because it requires a backspace character (a prohibited C0 control).
 Because RFC 5321 (and its predecessor, RFC 2821) prohibit the use of
 this character in ASCII mailbox names and it is even more problematic
 (for canonicalization and normalization reasons) in non-ASCII
 strings, backspace MUST NOT appear in SMTPUTF8 mailbox names.
 For the particular case of mailbox names that contain non-ASCII
 characters in the local part, domain part, or both, special attention
 MUST be paid to Unicode normalization [Unicode-UAX15], in part
 because Unicode strings may be normalized by other processes
 independent of what a mail protocol specifies (this is exactly
 analogous to what may happen with quoting and dequoting in
 traditional addresses).  Consequently, the following principles are
 offered as advice to those who are selecting names for mailboxes:
 o  In general, it is wise to support addresses in Normalized form,
    using at least Normalization Form NFC.  Except in circumstances in
    which NFKC would map characters together that the parties
    responsible for the destination mail server would prefer to be
    kept distinguishable, supporting the NFKC-conformant form would
    yield even more predictable behavior for the typical user.
 o  It will usually be wise to support other forms of the same local-
    part string, either as aliases or by normalization of strings
    reaching the delivery server: the sender should not be depended
    upon to send the strings in normalized form.

Klensin & Ko Standards Track [Page 16] RFC 6530 Internationalized Email Framework February 2012

 o  Stated differently and in more specific terms, the rules of the
    protocol for local-part strings essentially provide that:
  • Unnormalized strings are valid, but sufficiently bad practice

that they may not work reliably on a global basis. Servers

       should not depend on clients to send normalized forms but
       should be aware that procedures on client machines outside the
       control of the MUA may cause normalized strings to be sent
       regardless of user intent.
  • C0 (and presumably C1) controls (see The Unicode Standard

[Unicode]) are prohibited, the first in RFC 5321 and the second

       by an obvious extension from it [RFC5198].
  • Other kinds of punctuation, spaces, etc., are risky practice.

Perhaps they will work, and SMTP receiver code is required to

       handle them without severe errors (even if such strings are not
       accepted in addresses to be delivered on that server), but
       creating dependencies on them in mailbox names that are chosen
       is usually a bad practice and may lead to interoperability
       problems.

11. Additional Issues

 This section identifies issues that are not covered, or not covered
 comprehensively, as part of this set of specifications, but that will
 require ongoing review as part of deployment of email address and
 header internationalization.

11.1. Impact on URIs and IRIs

 The mailto: schema [RFC6068], and the discussion of it in the
 Internationalized Resource Identifier (IRI) specification [RFC3987],
 may need to be modified when this work is completed and standardized.

11.2. Use of Email Addresses as Identifiers

 There are a number of places in contemporary Internet usage in which
 email addresses are used as identifiers for individuals, including as
 identifiers to Web servers supporting some electronic commerce sites
 and in some X.509 certificates [RFC5280].  These documents do not
 address those uses, but it is reasonable to expect that some
 difficulties will be encountered when internationalized addresses are
 first used in those contexts, many of which cannot even handle the
 full range of addresses permitted today.

Klensin & Ko Standards Track [Page 17] RFC 6530 Internationalized Email Framework February 2012

11.3. Encoded Words, Signed Messages, and Downgrading

 One particular characteristic of the email format is its persistency:
 MUAs are expected to handle messages that were originally sent
 decades ago and not just those delivered seconds ago.  As such, MUAs
 and mail filtering software, such as that specified in Sieve
 [RFC5228], will need to continue to accept and decode header fields
 that use the "encoded word" mechanism [RFC2047] to accommodate non-
 ASCII characters in some header fields.  While extensions to both
 POP3 [RFC1939] and IMAP [RFC3501] have been defined that include
 automatic upgrading of messages that carry non-ASCII information in
 encoded form -- including RFC 2047 decoding -- of messages by the
 POP3 [RFC5721bis-POP3] or IMAP [RFC5738bis-IMAP] server, there are
 message structures and MIME content-types for which that cannot be
 done or where the change would have unacceptable side effects.
 For example, message parts that are cryptographically signed, using
 e.g., S/MIME [RFC5751] or Pretty Good Privacy (PGP) [RFC3156], cannot
 be upgraded from the RFC 2047 form to normal UTF-8 characters without
 breaking the signature.  Similarly, message parts that are encrypted
 may contain, when decrypted, header fields that use the RFC 2047
 encoding; such messages cannot be 'fully' upgraded without access to
 cryptographic keys.
 Similar issues may arise if messages are signed and then subsequently
 downgraded, e.g., as discussed in Section 8.1, and then an attempt is
 made to upgrade them to the original form and then verify the
 signatures.  Even the very subtle changes that may result from
 algorithms to downgrade and then upgrade again may be sufficient to
 invalidate the signatures if they impact either the primary or MIME
 body part headers.  When signatures are present, downgrading must be
 performed with extreme care if at all.

11.4. Other Uses of Local Parts

 Local parts are sometimes used to construct domain labels, e.g., the
 local part "user" in the address user@domain.example could be
 converted into a host name user.domain.example with its Web space at
 <http://user.domain.example> and the catch-all addresses
 any.thing.goes@user.domain.example.
 Such schemes are obviously limited by, among other things, the SMTP
 rules for domain names, and will not work without further
 restrictions for other local parts.  Whether those limitations are
 relevant to these specifications is an open question.  It may be
 simply another case of the considerable flexibility accorded to
 delivery MTAs in determining the mailbox names they will accept and
 how they are interpreted.

Klensin & Ko Standards Track [Page 18] RFC 6530 Internationalized Email Framework February 2012

11.5. Non-Standard Encapsulation Formats

 Some applications use formats similar to the application/mbox format
 [RFC4155] instead of the message/digest form defined in RFC 2046,
 Section 5.1.5 [RFC2046] to transfer multiple messages as single
 units.  Insofar as such applications assume that all stored messages
 use the message/rfc822 format described in RFC 2046, Section 5.2.1
 [RFC2046] with ASCII message headers, they are not ready for the
 extensions specified in this series of documents, and special
 measures may be needed to properly detect and process them.

12. Key Changes from the Experimental Protocols and Framework

 The original framework for internationalized email addresses and
 headers was described in RFC 4952 and a subsequent set of
 experimental protocol documents.  Those relationships are described
 in Section 3.  The key architectural difference between the
 experimental specifications and this newer set is that the earlier
 specifications supported in-transit downgrading.  Those mechanisms
 included the definition of syntax and functions to support passing
 alternate, all-ASCII addresses with the non-ASCII ones as well as
 special headers to indicate the downgraded status of messages.  Those
 features were eliminated after experimentation indicated that they
 were more complex and less necessary than had been assumed earlier.
 Those issues are described in more detail in Sections 6 and 9.

13. Security Considerations

 Any expansion of permitted characters and encoding forms in email
 addresses raises some risks.  There have been discussions on so
 called "IDN-spoofing" or "IDN homograph attacks".  These attacks
 allow an attacker (or "phisher") to spoof the domain or URLs of
 businesses or other entities.  The same kind of attack is also
 possible on the local part of internationalized email addresses.  It
 should be noted that the proposed fix involving forcing all displayed
 elements into normalized lowercase works for domain names in URLs,
 but not for email local parts since those are case sensitive.
 Since email addresses are often transcribed from business cards and
 notes on paper, they are subject to problems arising from confusable
 characters (see [RFC4690]).  These problems are somewhat reduced if
 the domain associated with the mailbox is unambiguous and supports a
 relatively small number of mailboxes whose names follow local system
 conventions.  They are increased with very large mail systems in
 which users can freely select their own addresses.

Klensin & Ko Standards Track [Page 19] RFC 6530 Internationalized Email Framework February 2012

 The internationalization of email addresses and message headers must
 not leave the Internet less secure than it is without the required
 extensions.  The requirements and mechanisms documented in this set
 of specifications do not, in general, raise any new security issues.
 They do require a review of issues associated with confusable
 characters -- a topic that is being explored thoroughly elsewhere
 (see, e.g., RFC 4690 [RFC4690]) -- and, potentially, some issues with
 UTF-8 normalization, discussed in RFC 3629 [RFC3629], and other
 transformations.  Normalization and other issues associated with
 transformations and standard forms are also part of the subject of
 work described elsewhere [RFC5198] [RFC5893] [RFC6055].
 Some issues specifically related to internationalized addresses and
 message headers are discussed in more detail in the other documents
 in this set.  However, in particular, caution should be taken that
 any "downgrading" mechanism, or use of downgraded addresses, does not
 inappropriately assume authenticated bindings between the
 internationalized and ASCII addresses.  This potential problem can be
 mitigated somewhat by enforcing the expectation that most or all such
 transformations will be performed prior to final delivery by systems
 that are presumed to be under the administrative control of the
 sending user (as opposed to being performed in transit by entities
 that are not under the administrative control of the sending user).
 The new UTF-8 header and message formats might also raise, or
 aggravate, another known issue.  If the model creates new forms of an
 'invalid' or 'malformed' message, then a new email attack is created:
 in an effort to be robust, some or most agents will accept such
 messages and interpret them as if they were well-formed.  If a filter
 interprets such a message differently than the MUA used by the
 recipient, then it may be possible to create a message that appears
 acceptable under the filter's interpretation but that should be
 rejected under the interpretation given to it by that MUA.  Such
 attacks already have occurred for existing messages and encoding
 layers, e.g., invalid MIME syntax, invalid HTML markup, and invalid
 coding of particular image types.
 In addition, email addresses are used in many contexts other than
 sending mail, such as for identifiers under various circumstances
 (see Section 11.2).  Each of those contexts will need to be
 evaluated, in turn, to determine whether the use of non-ASCII forms
 is appropriate and what particular issues they raise.
 This work will clearly affect any systems or mechanisms that are
 dependent on digital signatures or similar integrity protection for
 email message headers (see also the discussion in Section 11.3).
 Many conventional uses of PGP and S/MIME are not affected since they

Klensin & Ko Standards Track [Page 20] RFC 6530 Internationalized Email Framework February 2012

 are used to sign body parts but not message headers.  On the other
 hand, the developing work on DomainKeys Identified Mail (DKIM)
 [RFC5863] will eventually need to consider this work, and vice versa:
 while this specification does not address or solve the issues raised
 by DKIM and other signed header mechanisms, the issues will have to
 be coordinated and resolved eventually if the two sets of protocols
 are to coexist.  In addition, to the degree to which email addresses
 appear in PKI (Public Key Infrastructure) certificates [RFC5280],
 standards addressing such certificates will need to be upgraded to
 address these internationalized addresses.  Those upgrades will need
 to address questions of spoofing by look-alikes of the addresses
 themselves.

14. Acknowledgments

 This document is an update to, and derived from, RFC 4952.  This
 document would have been impossible without the work and
 contributions acknowledged in it.  The present document benefited
 significantly from discussions in the IETF EAI working group and
 elsewhere after RFC 4952 was published, especially discussions about
 the experimental versions of other documents in the internationalized
 email collection, and from RFC errata on RFC 4952 itself.
 Special thanks are due to Ernie Dainow for careful reviews and
 suggested text in this version and to several IESG members for a
 careful review and specific suggestions.

15. References

15.1. Normative References

 [ASCII]    American National Standards Institute (formerly United
            States of America Standards Institute), "USA Code for
            Information Interchange", ANSI X3.4-1968, 1968.
            ANSI X3.4-1968 has been replaced by newer versions with
            slight modifications, but the 1968 version remains
            definitive for the Internet.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, November 2003.
 [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            October 2008.

Klensin & Ko Standards Track [Page 21] RFC 6530 Internationalized Email Framework February 2012

 [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
            October 2008.
 [RFC5890]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Definitions and Document Framework",
            RFC 5890, August 2010.
 [RFC6152]  Klensin, J., Freed, N., Rose, M., and D. Crocker, "SMTP
            Service Extension for 8-bit MIME Transport", STD 71,
            RFC 6152, March 2011.
 [RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
            Email Address", RFC 6531, February 2012.
 [RFC6532]  Yang, A., Steele, S., and N. Freed, "Internationalized
            Email Headers", RFC 6532, February 2012.
 [RFC6533]  Hansen, T., Newman, C., and A. Melnikov,
            "Internationalized Delivery Status and Disposition
            Notifications", RFC 6533, February 2012.

15.2. Informative References

 [POPIMAP-downgrade]
            Fujiwara, K., "Post-delivery Message Downgrading for
            Internationalized Email Messages", Work in Progress,
            October 2011.
 [RFC0821]  Postel, J., "Simple Mail Transfer Protocol", STD 10,
            RFC 821, August 1982.
 [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
            and Support", STD 3, RFC 1123, October 1989.
 [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
            STD 53, RFC 1939, May 1996.
 [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part One: Format of Internet Message
            Bodies", RFC 2045, November 1996.
 [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part Two: Media Types", RFC 2046,
            November 1996.
 [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
            Part Three: Message Header Extensions for Non-ASCII Text",
            RFC 2047, November 1996.

Klensin & Ko Standards Track [Page 22] RFC 6530 Internationalized Email Framework February 2012

 [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
            Word Extensions: Character Sets, Languages, and
            Continuations", RFC 2231, November 1997.
 [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
            April 2001.
 [RFC3156]  Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
            "MIME Security with OpenPGP", RFC 3156, August 2001.
 [RFC3461]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
            Extension for Delivery Status Notifications (DSNs)",
            RFC 3461, January 2003.
 [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
            for Delivery Status Notifications", RFC 3464,
            January 2003.
 [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
            for Internationalized Domain Names in Applications
            (IDNA)", RFC 3492, March 2003.
 [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
            4rev1", RFC 3501, March 2003.
 [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
            Identifiers (IRIs)", RFC 3987, January 2005.
 [RFC4155]  Hall, E., "The application/mbox Media Type", RFC 4155,
            September 2005.
 [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
            Recommendations for Internationalized Domain Names
            (IDNs)", RFC 4690, September 2006.
 [RFC4952]  Klensin, J. and Y. Ko, "Overview and Framework for
            Internationalized Email", RFC 4952, July 2007.
 [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
            Interchange", RFC 5198, March 2008.
 [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering
            Language", RFC 5228, January 2008.
 [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, May 2008.

Klensin & Ko Standards Track [Page 23] RFC 6530 Internationalized Email Framework February 2012

 [RFC5335]  Yang, A., "Internationalized Email Headers", RFC 5335,
            September 2008.
 [RFC5336]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
            Email Addresses", RFC 5336, September 2008.
 [RFC5337]  Newman, C. and A. Melnikov, "Internationalized Delivery
            Status and Disposition Notifications", RFC 5337,
            September 2008.
 [RFC5504]  Fujiwara, K. and Y. Yoneya, "Downgrading Mechanism for
            Email Address Internationalization", RFC 5504, March 2009.
 [RFC5721]  Gellens, R. and C. Newman, "POP3 Support for UTF-8",
            RFC 5721, February 2010.
 [RFC5721bis-POP3]
            Gellens, R., Newman, C., Yao, J., and K. Fujiwara, "POP3
            Support for UTF-8", Work in Progress, November 2011.
 [RFC5738]  Resnick, P. and C. Newman, "IMAP Support for UTF-8",
            RFC 5738, March 2010.
 [RFC5738bis-IMAP]
            Resnick, P., Ed., Newman, C., Ed., and S. Shen, Ed., "IMAP
            Support for UTF-8", Work in Progress, December 2011.
 [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
            Mail Extensions (S/MIME) Version 3.2 Message
            Specification", RFC 5751, January 2010.
 [RFC5825]  Fujiwara, K. and B. Leiba, "Displaying Downgraded Messages
            for Email Address Internationalization", RFC 5825,
            April 2010.
 [RFC5863]  Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker,
            "DomainKeys Identified Mail (DKIM) Development,
            Deployment, and Operations", RFC 5863, May 2010.
 [RFC5891]  Klensin, J., "Internationalized Domain Names in
            Applications (IDNA): Protocol", RFC 5891, August 2010.
 [RFC5892]  Faltstrom, P., "The Unicode Code Points and
            Internationalized Domain Names for Applications (IDNA)",
            RFC 5892, August 2010.

Klensin & Ko Standards Track [Page 24] RFC 6530 Internationalized Email Framework February 2012

 [RFC5893]  Alvestrand, H. and C. Karp, "Right-to-Left Scripts for
            Internationalized Domain Names for Applications (IDNA)",
            RFC 5893, August 2010.
 [RFC5894]  Klensin, J., "Internationalized Domain Names for
            Applications (IDNA): Background, Explanation, and
            Rationale", RFC 5894, August 2010.
 [RFC5983]  Gellens, R., "Mailing Lists and Internationalized Email
            Addresses", RFC 5983, October 2010.
 [RFC5983bis-MailingList]
            Levine, J. and R. Gellens, "Mailing Lists and UTF-8
            Addresses", Work in Progress, December 2011.
 [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
            Encodings for Internationalized Domain Names", RFC 6055,
            February 2011.
 [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
            URI Scheme", RFC 6068, October 2010.
 [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
            STD 72, RFC 6409, November 2011.
 [Unicode]  The Unicode Consortium.  The Unicode Standard, Version
            6.0.0, defined by:, "The Unicode Standard, Version 6.0.0",
            (Mountain View, CA: The Unicode Consortium, 2011.  ISBN
            978-1-936213-01-6).,
            <http://www.unicode.org/versions/Unicode6.0.0/>.
 [Unicode-UAX15]
            The Unicode Consortium, "Unicode Standard Annex #15:
            Unicode Normalization Forms", September 2010,
            <http://www.unicode.org/reports/tr15/>.

Klensin & Ko Standards Track [Page 25] RFC 6530 Internationalized Email Framework February 2012

Authors' Addresses

 John C KLENSIN
 1770 Massachusetts Ave, #322
 Cambridge, MA  02140
 USA
 Phone: +1 617 491 5735
 EMail: john-ietf@jck.com
 YangWoo KO
 112-202 Malgeunachim APT. Nae-dong
 Seo-gu, Daejeon  302-981
 Republic of Korea
 EMail: yangwooko@gmail.com

Klensin & Ko Standards Track [Page 26]

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